The long-term reliability and safety of implanted medical leads is critical to the function of implanted medical devices. Conversely, lead anomalies constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). For example, early diagnosis of ICD lead anomalies is important to reduce morbidity and/or mortality from loss of pacing, inappropriate ICD shocks, and/or ineffective shock or pacing treatment of ventricular tachycardia (VT) or ventricular fibrillation (VF). Early diagnosis of anomalies in implanted cardiac leads is critical to improving reliability of ICD therapies.
Multilumen ICD defibrillation electrodes or leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted in a patient as subcutaneous, epicardial, or intravascular leads. Clinically, the most important lead failures have occurred in transvenous right ventricular (RV) defibrillation leads. These leads comprise a distal tip electrode with a fixation mechanism that anchors the lead to the right ventricle myocardium, proximal terminals that connect to the generator, and a lead body connecting the two. The “multilumen” lead body consists of a flexible, insulating cylinder with three to six parallel, longitudinal lumens through which conductors run from the proximal terminals to small pace-sense electrodes and larger shock coil electrodes. RV defibrillation leads have a distal shock coil in the right ventricle. The vast majority of presently-implanted transvenous ICD systems deliver therapeutic shocks between the right ventricle shock coil, having one polarity during the shock, and the housing (“CAN”) of the generator (“active” or “hot” CAN), which has the opposite polarity. Defibrillation leads may have either one or two shock coils and one or two dedicated sensing electrodes.
Dual coil vs. single coil leads: Dual-coil leads have an additional proximal shock coil, which typically lies in the superior vena cava (SVC). Dual-coil leads typically deliver shocks with the SVC shock coil electrically linked to the CAN and opposite in polarity to the RV shock coil. Alternatively, shocks may be delivered solely between the RV shock coil and SVC shock coil, without using the CAN as a shock electrode.
Integrated vs. true bipolar lead sensing configurations: Integrated-bipolar leads have a single sensing electrode on the tip. These leads sense the “integrated-bipolar” signal between the tip electrode and RV coil. True bipolar leads have an additional sensing-ring electrode. The sensing configuration can either be “true bipolar” between the tip electrode and a ring electrode or “integrated bipolar.”
Insulation breaches have been known to result in a functional failure of conductors within the lead or interactions among conductors of the same or different leads. Functional failure of a pace-sense conductor may result in symptoms caused by loss of pacing functions for bradycardia, cardiac resynchronization, or antitachycardia pacing. Functional failure of a high-voltage conductor may result in fatal failure of cardioversion or defibrillation. In addition, conductor interactions involving pace-sense conductors may result in over-sensing leading to inappropriate shocks or failure to pace. Interactions involving high-voltage electrodes may result in shorting the shock output, preventing life saving therapy from reaching the patient and potentially damaging the pulse generator irrevocably.
Insulation breaches or conductor fractures occur most commonly at two regions along the course of a defibrillation lead. The first region is within a pocket surgically created in a patient for implanting the implantable device, caused either by abrasion of the lead insulation by pressure from the housing (“CAN”) of the pulse generator (lead-CAN abrasion) or twisting and rubbing of the lead within the pocket against other elements of the same or a different lead (lead-lead abrasion). The second region is the intracardiac region that is between or under the shock coils in a dual-coil lead or proximal to the shock coil in a single coil leads. The second region is a common site of insulation breach for leads in the St. Jude Riata® family, for example, which are subject to “inside-out” insulation breach due to motion of the internal cables relative to the outer insulation. Multiple potential interactions are possible, including, but not limited to, inside-out abrasion of the cable to the RV shock coil against the proximal SVC shock coil, resulting in a short circuit within the lead. The lead may also be damaged between the clavicle and first rib, where the lead is subject to “clavicular crush,” usually resulting in conductor fracture.
Insulation breaches of ICD defibrillation leads positioned within the pocket in the patient can result in abrasion of the insulation around any of the cable conductors including the conductor to the RV coil, RV sensing ring, or SVC coil. One of the most dangerous lead failure conditions is abrasion of the insulation around the conductor of the RV coil (coil-CAN abrasion). This abrasion results in a short circuit between the CAN electrode and the RV coil, preventing defibrillation current from reaching the heart in the event of life threatening VT or VF. If a shock is delivered when this type of lead failure is present, extremely high current flowing through the shorted output circuit of the ICD may irrevocably damage the generator's components. (Hauser R G, McGriff D, Retel L K. Riata implantable cardioverter-defibrillator lead failure: analysis of explanted leads with a unique insulation defect. Heart Rhythm. 2012; 9:742-749; Hauser R G, Abdelhadi R H, McGriff D M, Kallinen Retel L. Failure of a novel silicone-polyurethane copolymer (Optim) to prevent implantable cardioverter-defibrillator lead insulation abrasions. Europace. 2013; 15:278-283.)
ICDs can contain circuits that protect the electrical integrity of the generator against shorted high voltage outputs during delivery of a shock. These circuits abort the shock if the current in the output circuit is sufficiently high, which can be indicative of a short circuit diverting current from the heart. Thus, although such protective circuitry may prevent damage to the generator, the potentially lifesaving shock fails to be delivered to the patient. U.S. Pat. No. 7,747,320 to Kroll teaches a backup defibrillation mode method which exclude shorted electrodes during a shock. However, this method applies only during shock delivery of a high output shock in response to detection of VF or VT by the ICD, cannot be used with single coil leads and can result in shock delivery through only part of the intended defibrillation pathway, with unknown defibrillation efficacy. Further, such a high output shock still may have enough energy prior to aborting shock delivery to ablate additional insulation which will exacerbate the insulation breach and potentially even “spot weld” the exposed conductor to the housing, exacerbating the short circuit.
Unfortunately, detection of lead anomalies in implanted medical leads prior to delivery of an electrical therapy, for example, a high voltage shock, involves trying to simultaneously achieve at least two different goals. One goal is high sensitivity of diagnosis for the identification of lead failures at the subclinical stage, before they present as a clinical problem. A second goal is high specificity because a false positive provisional clinical diagnosis of lead failure may trigger patient anxiety and lead to potentially avoidable diagnostic testing. A false positive clinical diagnosis of lead failure may result in unnecessary lead replacement, with corresponding expense and surgical risk. Balancing these goals to achieve an effective identification of lead anomalies without an unacceptable number of false positives has been difficult to accomplish.
Existing technology for diagnosis of lead anomalies in an implanted ICD lead is believed to have significant limitations and shortcomings, especially with regard to diagnosis of high-voltage insulation breaches prior to shock delivery. ICDs routinely deliver low voltage pulses, on the order of about 1.0 volts to about 15.0 volts, or switched AC pulse trains to measure the impedance of the high voltage shock pathway. However, these low-voltage measurements of shock-electrode impedance may not identify insulation breaches in which the insulation's dielectric properties remain intact at low voltages but break down during high-voltage shocks. Clinical case reports indicate that high-voltage insulation breaches may not be detected by the low voltage measurements, and, despite nominal values of such measurements, high voltage clinical shocks have short circuited, preventing the current from reaching the heart and defibrillating VF. (Shah P, Singh G, Chandra S, Schuger C D. Failure to deliver therapy by a Riata lead with internal wire externalization and normal electrical parameters during routine interrogation. J Cardiovasc Electrophysiol. 2013; 24:94-96.)
Existing technology for diagnosis of anomalies in implanted pacemaker leads and low voltage lead components is also believed to have significant limitations and shortcomings, especially with regard to early diagnosis. The primary method currently in use for monitoring pacemaker lead integrity is periodic measurement of electrical resistance, commonly referred to as “impedance monitoring.” Impedance monitoring uses single pulses. Various methods are well-known and provide a value of impedance close to the direct current resistance.
In the circuit being measured, most of the resistance is at the electrode-tissue interface of the high-resistance tip electrode where variations of up to 10% in the resistive value are common. Each individual pace-sense conductor (for example, the conductor to the tip electrode or the ring electrode) contributes less than 10% to the measured resistance. Thus even if the resistance in a single conductor doubled or tripled, the overall measured resistance will remain within the expected range. Measurements indicate that resistance does not exceed the expected range until the conductor has lost most of its structural integrity. Thus, resistance measurements are insensitive to partial loss of conductor integrity. Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked increases can occur at the electrode-myocardial interface.
Hafelinger et al. (U.S. Pat. No. 5,003,975) and Cinbis et al. (U.S. Pat. No. 5,897,577) summarize some of these methods, which include measurements made directly using either a single pacing pulse or a single independent pulse used only for measuring resistance. McVenes et al. (U.S. Pat. No. 5,741,311) describes use of a longer (about 100 ms) burst of alternating current at a single frequency to drive the system to a steady-state condition that is not achieved by single, short (less than 1 ms) pacing pulses. Schuelke et al. (U.S. Pat. No. 5,755,742) describes a method for measuring resistance of defibrillation electrodes by applying a current to a parallel pathway. Kroll et al. (U.S. Pat. No. 5,944,746) describes an automated method for periodic measurement of the resistance of the high-voltage (defibrillating) coil in ICD electrodes. Gunderson et al. (U.S. Pat. No. 7,047,083) describes a method and system for automated, periodic measurements of resistance in conductors attached to an ICD or pacemaker. However, these methods identify lead anomalies before inappropriate shocks in only about a third of ICD patients who have conductor fractures and an even lower fraction with insulation breaches (Swerdlow C D, Gunderson B D, Ousdigian K T, Abeyratne A, Sachanandani H, Ellenbogen K A., Downloadable software algorithm reduces inappropriate shocks caused by implantable cardioverter-defibrillator lead fractures: a prospective study. Circulation. 2010; 122: 1449-1455) (Sung R K, Massie B M, Varosy P D, Moore H, Rumsfeld J, Lee B K, Keung E., Long-term electrical survival analysis of Riata and Riata ST silicone leads: National Veterans Affairs experience. Heart Rhythm. 2012; 9:1954-1961.) (Ellenbogen K A, Gunderson B D, Stromberg K D, Swerdlow C D., Performance of Lead Integrity Alert to assist in the clinical diagnosis of implantable cardioverter defibrillator lead failures: analysis of different implantable cardioverter defibrillator leads. Circ Arrhythm Electrophysiol. 2013; 6:1169-1177.)
A different method for monitoring defibrillation lead sensing integrity is based on sensing of rapid non-physiological signals associated with lead conductor fractures. Frei (U.S. Pat. No. 7,146,211) describes detection of saturation artifacts from intracranial electrodes indicative of noise as an indication of poor connections or open conductors. Repetitive over-sensing of non-physiologically short intervals may indicate lead conductor fracture even if lead resistance is normal. Gunderson et al. (U.S. Pat. No. 7,289,851) described a Lead-Integrity Alert that incorporates both ICD-based measures of over-sensing based on the non-physiologically rapid rate of sensed signals and periodic measurements of resistance. This method, combined with automatic ICD reprogramming, improves warning time before inappropriate shocks caused by lead-related over-sensing. Nevertheless, approximately 40% of patients receive inappropriate shocks if the implanted lead has a conductor fracture. (Swerdlow C D, Gunderson B D, Ousdigian K T, Abeyratne A, Sachanandani H, Ellenbogen K A. Downloadable software algorithm reduces inappropriate shocks caused by implantable cardioverter-defibrillator lead fractures: a prospective study. Circulation. 2010; 122:1449-1455.) In addition to limited sensitivity, present methods for diagnosing lead anomalies have limited specificity resulting in false positive diagnostics. (Ellenbogen K A, Gunderson B D, Stromberg K D, Swerdlow C D. Performance of Lead Integrity Alert to assist in the clinical diagnosis of implantable cardioverter defibrillator lead failures: analysis of different implantable cardioverter defibrillator leads. Circ Arrhythm Electrophysiol. 2013; 6:1169-1177.) Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks, and clinical reports document that this has happened. (Swerdlow C D, Sachanandani H, Gunderson B D, Ousdigian K T, Hjelle M, Ellenbogen K A., Preventing overdiagnosis of implantable cardioverter-defibrillator lead fractures using device diagnostics. J Am Coll Cardiol. 2011; 57:2330-2339.)
Gunderson et al. (U.S. Pat. No. 7,369,893) further describes a method for withholding delivery of ICD shocks if VF is detected from analysis of the pace-sense lead, but not confirmed by analysis of the high-voltage lead. St. Jude Medical has introduced an algorithm (“SecureSense®”) that incorporates features similar to those described in U.S. Pat. No. 7,369,893. The presumption is that these signals do not represent true cardiac activations. However, this method requires sufficient over-sensing of spontaneously-generated, unpredictable, rapid non-cardiac signals to cause inappropriate detection of VF clinically. Thus, it does not provide early diagnosis of conductor anomalies. Further, withholding shocks for VF detected on the near-field electrogram has an inherent risk of withholding life-saving therapy, however small, if a false positive test outcome occurs. As such, it is not the preferred approach to diagnosis conductor fracture.
Each of the passive monitoring algorithms discussed above identifies lead anomalies in implanted leads based on sensing of rapid, non-physiological signals, and suffer from two major, inherent limitations. First, they depend on the uncontrollable and unpredictable occurrence of non-physiological signals on the sensing channel due to lead anomalies. Second, they cannot discriminate such signals from other rapid, over-sensed signals.
Detection of physiological signals to aid in the identification of potential lead anomalies also has been suggested. Gunderson (U.S. Patent Publication No. 2010/0023084) describes detecting saturation of a physiological signal on an electrode associated with an implantable medical lead. Gunderson (U.S. Patent Publication No. 2011/0054558) describes delivering a pacing stimulus or other test stimulus through a pace-sense channel and subsequently monitoring the same channel for the occurrence of anomalous signals. Unfortunately, none of these approaches for detection of physiological signals has proven effective in identifying lead anomalies with both high sensitivity and high specificity.
What is desired is a method and apparatus to provide sensitive and specific diagnosis of lead anomalies in implanted medical leads at the subclinical stage, that does not depend on the unpredictable occurrence of anomalous signals, and that can apply to all kinds of implanted medical leads and their component conductors, including both lower voltage conductors such as pace-sense components or leads and higher voltage defibrillation components.